The present invention relates generally to electrochemical cells, and particularly to zinc-chlorine batteries combined to form a battery plant system.
Due to the increasing demand for electricity and the decreasing availability (and increasing cost) of distillate oil and natural gas, the need has arisen for an alternate method of supplying peak demand electricity. Presently, the electricity generated for peak demand is supplied from diesel engines and combustion turbines, which are fired by distillate oil and natural gas. One such method is the use of secondary energy storage batteries to store electricity generated from utility baseload facilities during the night or off-peak hours, and discharging these batteries during the hours of peak demand. Secondary energy storage batteries currently being considered for this application include lead-acid, lithium-iron sulfide, sodium sulfur, sodium-chloride, and zinc-chlorine batteries. In order to be utilized in this application, these batteries would necessarily have to be scaled up to battery plants capable of delivering electrical energy on the order of 100 mega-watt hours in a single discharge. This scale up would generally be achieved by combining large numbers of cells into module-type units, and interconnecting a suitable number of these modules.
One of the primary concerns in such a scale up, is the reliability of the battery plant. This reliability may generally be characterized as a function of the number of battery module failures. Since these modules would usually be connected electrically in series to form battery strings, the failure of a single module will affect the operation of the entire string. If the failure is such that the battery string must be disconnected from the electrical current flow in the battery plant, this has the effect of the failure of all of the battery modules in the string.
A bypass switch for redirecting electrical current around a failed battery module is described in the commonly assigned U.S. Pat. No. 4,287,267, entitled "Zinc-Chlorine Battery Plant System And Method", issued on Sept. 1, 1981 to Whittlesey et al., which is hereby incorporated by reference. This bypass switch permits only the failed battery module or modules to be effectively removed from the battery string by short circuiting the electrical power terminals of the failed modules(s).
Accordingly, the provision of a bypass switch in association with each battery module in the battery plant considerably enhances the reliability of the battery plant by enabling the battery string containing a failed module to continue to charge or discharge with the other battery strings. However, the removal of one or more battery modules in this way will give rise to an undesirable electrical current balance between the battery strings which are connected electrically in parallel by virtue of the lower voltage across the battery string having the failed module with respect to the voltages across the other battery strings. For example, if each battery string contained 20 battery modules each operating at 50 volts, the normal voltage across a battery string would be 1000 volts. If one of the battery modules failed and was removed from the battery string via a bypass switch, the instantaneous voltage of the battery string affected would be 950 volts. However, since all of the battery strings are connected electrically in parallel, a redistribution of the electrical current flow through the battery strings would automatically result in an effort to equalize the voltages across each of the battery strings. This redistribution of electrical current flow would mean a significant increase in the current flow in the battery string having the failed battery module with a concomitant decrease in the current flow through the remaining battery strings.
In the U.S. Pat. No. 4,287,267, this current imbalance problem was resolved by providing a separate converter bridge for each battery string so that variations in a battery string voltage could be compensated for by changing the thyristor firing angles. However, in accordance with the present invention, a magnetic technique is employed to balance the electrical current flow through the battery strings which does not require the provision of separate converter bridges for each battery string. Additionally, in an alternative embodiment a single high voltage converter is provided, and a low voltage converter associated with each battery string is used to balance the current flow.
Although the description below is directed specifically to zinc-chloride batteries, the present invention may also be utilized with other types of metal-halogen batteries and electrochemical systems which employ a plurality of battery strings which are connected electrically in parallel. With general reference to metal-halogen battery systems, these battery systems are generally comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metalhalide electrolyte. The electrolyte circulation subsystem operates to circulate the metalhalide electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte ionic components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive or "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the store temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, were it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. A further description may be found in the commonly assigned U.S. co-pending patent application entitled "A Hydrogen Gas Relief Valve" by Curtis C. Whittlesey, filed on July 8, 1983, Ser. No. 510,401. An additional technique for resolving the current imbalance problem is also disclosed in the commonly assigned U.S. patent application Ser. No. 515,348, entitled "Device for Balancing Parallel Strings", by Matthew S. Mashikian which was filed on even date herewith. The specific teachings of the aforementioned cited references are incorporated herein by reference.
It is a principal objective of the pesent invention to provide an apparatus and method of balancing the electrical current flow through a plurality of battery strings which are connected electrically in parallel across common bus conductors and in which each of the battery strings is formed by a plurality of batteries connected electrically in series.
It is another objective of the present invention to provide a battery plant which will magnetically control the voltage across the common bus conductors for each battery string.
It is a further objective of the present invention to provide a battery plant which will magnetically prevent a redistribution of the electrical current flow through the battery strings in response to a failed battery in one or more of the battery strings.
It is an additional objective of the present invention to provide a battery plant which will magnetically prevent a redistribution of the electrical current flow through the battery strings in response to the removal of one or more batteries from a battery string.
It is yet another objective of the present invention to provide a battery plant which utilizes low voltage converters to balance the electrical current flow through the battery strings.
To achieve the foregoing objectives, the present invention provides a battery plant which features magnetic circuit means for balancing the electrical current flow through a plurality of battery strings which are connected electrically in parallel. The magnetic circuit means is associated with the battery strings such that the conductors carrying the electrical current flow through each of the battery strings pass through the magnetic circuit means in directions which cause the electromagnetic fields of at least one predetermined pair of the conductors to oppose each other. In an alternative embodiment, a low voltage converter is associated with each of the battery strings for balancing the electrical current flow through the battery strings.